It is understood that the MRSA did not show up when the patients were screened on their arrival at hospital.

But the bug was found a day later following their operations.

The theatre where the surgery had taken place was immediately sealed off to new non-urgent cases to allow the cleaning to be carried out and an investigation launched.

The hospital has been MRSA free for 201 days – and that record remains intact because the bug had been detected before entering the patients' bloodstream and causing septicaemia.

The restrictions on all but emergency operations stayed for several days but have now been lifted.

The cases come after two new-born babies died and a third is still in isolation at the University Hospital following a different infection at its intensive care neo-natal unit in early July.

Emyr Phillips, the University Hospital's lead infection prevention nurse, said: "We identified a small number of cases of patients carrying MRSA following breast surgery.

"Although it is found on the skin of 30 per cent of the general population we would not expect it to be concentrated on such a select group of seven patients.

"Through our rigorous screening programme we were able to detect it and prevent it getting into the blood to cause bacteraemia.

"All seven were treated with intravenous antibiotics and have now been discharged to their homes.

"We have now gone more than 210 days without MRSA and our clinical staff are dedicated to ensuring this successful fight against avoidable infections continues."

It has emerged that not all of the breast operations involved were for breast cancer, although some of the routine procedures subsequently cancelled could have been to tackle the disease.

Dr Ken Deacon, medical director of the area's primary care trusts which check on safety at the University Hospital, said: "We cannot say how the seven patients showed no sign of the MRSA on admission but were found with it following their operations.

"But we are satisfied that with both this, and the outbreak in the neo-natal intensive care unit, the hospital did all it could to contain it."

Around a decade ago, the University Hospital was dealing with around 100 MRSA cases every year.

A hospital spokesman said: "These seven patients may not have been identified back then and therefore could have gone on to develop the illness itself."